Electronic structures and carrier transport mechanisms in disordered oxide semiconductors, crystalline InGaO3(ZnO)m (m = 1,5) (c-IGZO)and amorphous InGaZnO4 (a-IGZO), are examined based on a percolation conduction model. Donor levels (Ed) and densities (ND) are estimated by numerical calculations of free electron densities (ne) obtained by Hall measurements. It shows that the donor levels are rather deep, ~0.15 eV for c-IGZO and ~0.11 eV for a-IGZO. This analysis indicates that use of a simple analytical relation of ne exp(-Ed/2kT can not always be used to estimate Ed and ND even for a low ne film because the film can be in the saturation regime at room temperature if Ed and ND are small, which is actually the case for a-IGZO. The temperature dependences of electron mobilities are analyzed using an analytical equation of the percolation conduction model, which reveals that distributed potential barriers exist above mobility edges in IGZO with average heights 30–100 meV and distribution widths 5–20 meV, which depend on atomic structure and deposition condition of IGZO films. High-quality a-IGZO films have the lowest potential barriers among the IGZO films examined, in spite that a-IGZO has a more disordered amorphous structure than c-IGZO have. It is explained by the partly disordered structure of c-IGZO.
© 2009 IEEE
Toshio Kamiya, Kenji Nomura, and Hideo Hosono, "Electronic Structures Above Mobility Edges in Crystalline and Amorphous In-Ga-Zn-O: Percolation Conduction Examined by Analytical Model," J. Display Technol. 5, 462-467 (2009)